Code Calculations

The NEC requirements for sizing and installing boxes, conduit bodies, and fittings help prevent damage to conductor insulation during installation and subsequent maintenance. What are those requirements, and how will you be sure you meet them? To answer those questions, turn to Art. 314 of the NEC, which consists of the following five parts: Part I Scope and General. This section of the Code sets

The NEC requirements for sizing and installing boxes, conduit bodies, and fittings help prevent damage to conductor insulation during installation and subsequent maintenance. What are those requirements, and how will you be sure you meet them? To answer those questions, turn to Art. 314 of the NEC, which consists of the following five parts:

Part I — Scope and General. This section of the Code sets forth some basic rules on the installation and use of all boxes and conduit bodies used as outlet, device, junction, or pull boxes, as well as manholes and other electric enclosures intended for personnel entry.

Part II — Installation. Because of its extensive coverage of box and body fill calculations, Part II makes up the bulk of Art. 314. In addition to all the detailed text, it contains Tables 314.16(A) and (B), which you should refer to when calculating these box sizes.

Part III — Construction Specifications. Little of what appears in Part III concerns the typical electrician — if you use listed and labeled equipment, you're covered. Just be aware of the provisions, and don't modify listed or labeled equipment.

Part IV — Manholes and Other Electric Enclosures Intended for Personnel Entry. As with Part III, be aware of the provisions and don't modify listed and labeled equipment.

Part V — Systems Over 600V. The major concerns in this section are maintaining distances and clearances from other equipment.

When using conductors 4 AWG and larger, you must size pull boxes, junction boxes, and conduit bodies per 314.28. Let's dig right in and see what 314.28 requires, based on the necessary type of calculation.

Straight-pulls [314.28(A)(1)]. A straight-pull calculation applies when conductors enter one side of a box and leave through the opposite wall of the box. The minimum distance from where the raceway enters to the opposite wall must not be less than eight times the trade size of the largest raceway (Fig. 1).

Angle-pulls [314.28(A)(2)]. An angle-pull calculation applies when conductors enter one wall and leave the enclosure not opposite the wall of the conductor entry. The distance for angle-pull calculations from where the raceway enters to the opposite wall must not be less than six times the trade diameter of the largest raceway, plus the sum of the diameters of the remaining raceways on the same wall and row (Fig. 2).

That sounds a bit complicated, so let's put it another way. If you apply the following variables:

E=Entry point of raceway O=Opposite wall D=Trade diameter S=Sum of the diameters of the remaining raceways on the same wall and row

Then the minimum distance from E to O=6ÛD+S

When there is more than one row or raceway, you must calculate each row separately. Use the largest calculated value for your minimum angle-pull dimension.

U-pulls [314.28(A)(2)]. A U-pull calculation applies when the conductors enter and leave from the same wall. The distance from where the raceways enter to the opposite wall must not be less than six times the trade diameter of the largest raceway, plus the sum of the diameters of the remaining raceways on the same wall. Fig. 3 above helps clarify this concept.

Distance between raceways containing the same conductor calculation [314.28(A)(2)]. After sizing the pull box, the raceways must be installed so the distance between raceways that enclose the same conductors is not less than six times the trade diameter of the largest raceway. You measure this distance from the nearest edge of one raceway to the nearest edge of the other raceway (Fig. 4).

Depth of box and conduit body sizing [314.28(A)(2), Ex.]. When conductors enter an enclosure opposite a removable cover, such as the back of a pull box or conduit body, the distance from where the conductors enter to the removable cover shall not be less than the distances listed in Table 312.6(A); one wire per terminal (Fig. 5).

Let's work through an example to try out the material covered so far.

Example No. 1: Two 2-inch conduits that contain 4/0 AWG conductors enter the back of a 24-inch by 24-inch pull box. What is the minimum depth of the box?

(a) 4 inches (b) 6 inches (c) 8 inches (d) 10 inches

Per Table 312.6(A), the correct answer is (a), 4 inches.

Junction and pull box sizing tips. When sizing pull and junction boxes, the following suggestions should be helpful:

Step 1: Draw out the problem on paper so you can visualize it. This reduces error and usually speeds up the process.

Step 2: Calculate the horizontal distance(s):

Left to right straight calculation

Left to right angle or U-pull calculation

Right to left straight calculation

Right to left angle or U-pull calculation

Step 3: Calculate the vertical distance(s):

Top to bottom straight calculation

Top to bottom angle or U-pull calculation

Bottom to top straight calculation

Bottom to top angle or U-pull calculation

Let's now apply what we've covered so far so you can see how this process of correct calculation works. As you do some sample exercises, the knowledge you've gained should really begin to stick.

Example No. 2: A junction box contains two 3-inch raceways on the left side and one 3-inch raceway on the right side. The conductors from one of the 3-inch raceways on the left wall are pulled through a 3-inch raceway on the right wall. The conductors from the other 3-inch raceway on the left wall are pulled through a 3-inch raceway at the bottom of the pull box (Fig. 6).

What is the horizontal dimension of this box?

(a) 18 inches (b) 21 inches (c) 24 inches (d) 30 inches

First, determine the left to right angle pull: (6×3 in.)+3 in.=21 in.

Then, calculate the left to right straight pull: 8×3 in.=24 in.

Next, check the right to left angle pull: No calculation required.

Finally, determine the right to left straight pull: 8×3 in.=24 in.

Based on these calculations, the answer is (c), 24 inches.

What is the vertical dimension of this box?

(a) 18 inches (b) 21 inches (c) 24 inches (d) 30 inches

First, check the top to bottom angle pull: No calculation required.

Then, check the top to bottom straight pull: Again, no calculation required.

Next, calculate the bottom to top angle pull: 6×3 in.=18 in.

Finally, determine the bottom to top straight pull: Again, no calculation required.

The answer is (a), 18 inches.

What is the minimum distance between the two 3-inch raceways that contain the same conductors?

(a) 18 inches (b) 21 inches (c) 24 inches (d) 28 inches

The angle pull is the only applicable calculation here: 6×3 in.=18 in.

Therefore, the answer is (a), 18 inches.

Example No. 3: A pull box contains two 4-inch raceways on the left side and two 2-inch raceways on the top.

What is the horizontal dimension of the box?

(a) 28 inches (b) 21 inches (c) 24 inches (d) 30 inches

The left to right angle pull calculation is the only applicable step here: (6×4 in.) +4 in.=28 in.

What is the minimum distance between the two 4-inch raceways that contain the same conductors?

(a) 18 inches (b) 21 inches (c) 24 inches (d) 28 inches

The angle pull is the only applicable calculation here: 6×4 inches=24 in.

Therefore, the correct answer is (c), 24 inches.

To strengthen your ability in this area, rework the examples above, but change some numbers. For example, size a pull box that contains two 3-inch raceways on the left side and three 2-inch raceways on the top. Don't forget to draw out the problem. Then you can see just how good you're getting as you practice.

Putting it all together. By now, you should feel fairly confident in sizing boxes, enclosures, and fittings. Art. 314 can appear intimidating at first. However, once you understand some basics and work through some examples, you can correctly size these items every time.